Active matrix organic light emitting diode (AMOLED) displays are becoming more main stream, due to their better contrast ratios when compared to conventional liquid crystal displays (LCDs). AMOLED displays are self-emissive devices, and do not require backlights. AMOLED displays may also provide more vivid colors and a larger color gamut than the conventional LCDs. Further, AMOLED displays can be made more flexible, thinner, and lighter than a typical LCD.
An OLED generally includes an anode, one or more organic layers, and a cathode. AMOLEDs can either be a bottom emission OLED or a top emission OLED. In bottom emission OLEDs, the light is extracted from an anode side. In such embodiments, the anode is generally transparent, while a cathode is generally reflective. The pixel area is shared between the OLED and a backplane driving circuit that generally includes one or more thin film transistors (TFT), metal routings, capacitors. The driving circuit may contain several TFTs, signal traces for control signals, and one or more capacitors. As a result, the pixel area may be limited and the corresponding OLED aperture for light emission may likewise be limited, as the aperture generally overlies the pixel area but not the driving circuit.
In a top emission OLED, light is extracted from a cathode side. The cathode is optically transparent, while the anode is reflective. This top emission OLED normally enables a larger OLED aperture than a bottom emission OLED, since the top emission OLED is fabricated on top of TFT.
In a top emission OLED, the common electrode may be formed from a transparent conductive material like indium-tin-oxide (ITO) and/or thin metals such as magnesium and silver. A metal common electrode may have better electrical conductivity than a common electrode formed from ITO, but to the metal common electrodes generally must be very thin in order to be optically transparent or semi-transparent. Such thin metal layers make the sheet resistance of the common electrode relatively large, especially when compared to a common electrode of a bottom emission OLED. Since light does not travel through the common electrode in a bottom emission OLED, the common electrode does not need to be optically transparent and so can be made as thick as desired. Accordingly, the bottom emission OLED may have a much smaller sheet resistance than the top emission OLED. It is desirable to reduce sheet resistance of the common OLED electrode in the top emission OLEDs such that less power may be required from a power supply to operate the OLEDs.